(i) Field of the Invention
This invention relates to poles and their production, more especially it is concerned with utility or distribution poles which may be used to support overhead lines in power transmission and in external lighting of different kinds, for example, street and highway lighting.
(ii) Brief Description of the Prior Art
Most of the utility poles in use are wood poles. Wood poles are light, easy to erect, have acceptable durability and are relatively inexpensive.
It is expected that in future, the supply of wood poles will not meet the demand for poles. In addition, while wood poles are satisfactory for use in rural areas, where they blend with the rural landscape and environment, and there is an abundance of space for guying, they are generally considered unsatisfactory in urban areas where space is limited and where they do not blend architecturally with the urban landscape and environment.
Concrete poles have been employed in urban areas, which poles have an aesthetically pleasing appearance in the urban landscape and blend in with their surroundings. Concrete poles are reinforced, typically with steel bars or rods, to provide adequate strength and deflection characteristics.
The reinforcing steel bars or rods in concrete poles are subject to the corrosive action of water containing dissolved components found in the environment such as salts and acidic gases, which water penetrates through the pores of the concrete.
In view of this it is necessary to provide a concrete pole with a thick concrete wall, typically more than 2 in. thick, in order to protect the reinforcing rods against such corrosion, and provide a pole with a satisfactory life. This need for a thick concrete wall results in a pole which is about twice the weight of a wood pole of comparable length. The greater weight of the conventional reinforced concrete poles increases the cost of installation relative to wood poles and, in particular, requires the use of more expensive equipment.
Reinforced concrete has been employed in products for outdoor use for over 100 years. Practical rules have evolved for such reinforced concrete in order that products have an acceptable working life. These rules, including those which govern poles, are set forth in A.C.I. (American Concrete Institute) 318-83 Building Code Requirements for Reinforced Concrete which specifies:
(i) The reinforcement, except for spirals and tendons, shall be deformed bars (Clause 3.5.1.). PA1 (ii) Designs may not use a yield strength exceeding 80,000 psi (Clause 9.4.). PA1 (iii) Concrete cover over the reinforcing bars shall not be less than 1.25 inches (Clause 7.7.2(a).).
These requirements arise from the fact that, on the one hand, concrete cracks at low levels of tensile stress and, for strength, the tensile zones of concrete structural members must be reinforced with steel while, on the other hand, exposed steel corrodes easily and, for durability, it must be protected by the concrete. Since the protection provided by the concrete depends, in part, on its thickness and in part on the size of cracks in it, it has been found necessary to employ a minimum cover of concrete, depending on the exposure, and to limit crack sizes by limiting the shape and strength of reinforcement.
Concrete poles have the same durability needs as other concrete products. Indeed, those poles placed adjacent to salted roadways and along salt-water shore lines experience an even more aggressive environment than the average outdoor structure and should preferably be subject to greater durability measures than the minimum.
While early concrete poles were wet-cast, most poles in North America are now made by the spin-casting process, which allows the production of a hollow shell of concrete of circular or polygonal shape. The theoretical minimum wall thickness of such a shell is governed by the requirement that the cover on each side must be at least 3/4 inch while the smallest deformed reinforcement bar has a diameter of 3/8 inch, for a total thickness of 17/8 inch. Since concrete has approximately five times the density of wood, and since the external dimensions of concrete and wood poles are similar, it can readily be shown that durable hollow concrete poles must, theoretically, weigh more than twice as much as a solid wood pole. In practice, since reinforcing bars are more commonly required to be 1/2 inch or 5/8 inch in diameter, and since many poles need to incorporate reinforcing spirals, wound over the axial reinforcement; and since some allowance for construction tolerance must be maintained, most concrete poles have shell thicknesses greater than 2 inches and have a weight closer to three times the weight of the corresponding wood pole. For example, a typical 45 foot long wood distribution pole would weigh about 900 lbs. (with a modest variation for class and species) while the corresponding concrete pole would weigh about 2,700 lbs.
Since most pole users have moving and erection equipment proportioned to the more commonly used wood pole, it is clear that the substantial weight difference is an economic burden and an operational nuisance, and a weight closer to that of wood is highly desirable.
Furthermore, since concrete poles are more commonly used in urban areas, and since pole replacement on busy streets is both costly and a traffic hazard, it is desirable that urban poles should have a longer life than either the wood poles or the concrete poles currently in use, and the conventional way of increasing durability by increasing the wall thickness and thus the weight is clearly objectionable.
Swiss Pat. No. 179,366 issued Sept. 15, 1935, Gustav E. Vogt, teaches a hollow concrete pole reinforced with reinforcing rods and having an irregular polygon as its cross section, in which alternate sides contain reinforcement and are of conventional thickness while the intermediate sides contain no reinforcement and can therefore be made thinner.
According to Vogt, the thinning of some sides, made possible by omitting the reinforcement in them, leads to a desirable saving in weight. However, a consequent disadvantage of Vogt is that the pole may not be tapered, which increases the weight as compared with a tapered pole and, because only half the perimeter is available for reinforcement, the number of bars that can be contained must be either less than can be contained in a conventional pole or else the total perimeter must be increased by increasing the diameter of the pole.
Attempts have also been made to protect the steel reinforcing rods without the need for a thick concrete wall, for example, by galvanizing to provide a sacrificial coating of zinc on the steel surface. However, the resulting poles do not have a satisfactory life.
It is an object of this invention to overcome the disadvantages of conventional reinforced concrete poles, while at the same time providing a pole having the aesthetic appearance associated with the reinforced concrete pole.
It is a further object of this invention to provide a pole which is light in weight, as compared with a reinforced concrete pole, and yet is durable and has a long life.
It is still another object of this invention to provide a thin walled, tapered, tubular pole having good deflection characteristics.
It is yet another object of this invention to provide a pole which is concrete-like in appearance and performance and yet has a weight substantially less than that of a conventional concrete pole and closer to that of a wood pole.
It is still a further object of this invention to provide a pole of increased durability without sacrificing the saving in weight.
It is yet a further object of this invention to achieve these effects by combining methods and materials which substantially reduce the thickness of pole wall which can be used.
It is yet another object of this invention to provide a method of manufacturing a pole.